Low loss thermoelectric heat exchanger



Nov. 9, 1965 N. P. MILLIGAN ETAL 3,216,205

LOW LOSS THERMOELECTRIC HEAT EXCHANGER 2 Sheets-Sheet 1 Original FiledJan. 15, 1963 zzvmvro NEAL P. lvllLusAlv MIAMES P. BURGESS 764 04 Mina;

ATTORNEYS Nov. 9, 1965 N. P. MILLIGAN ETAL LOW LOSS THERMOELECTRIC HEATEXCHANGER Original Filed Jan. 15, 1963 2 Sheets-Sheet 2 INVENTORS NEALP. MILLIGAN S= /XMES P. BURGESS A TTORNEYS 'is not energized.

United States. Patent 3,216,205 LOW LOSS THERMOELECTRIC HEAT EXCHANGERNeal P. Milligan and James P. Burgess, Columbus, Ohio, assignors toTecumseh Products Company, Tecumseh, Mich., a corporation of MichiganOriginal application Ser. No. 251,618, Jan. 15, 1963. Divided and thisapplication Dec. 31, 1964, Ser. No. 435,104

6 Claims. (Cl. 623) This application is a division of our co-pendingapplication Serial No. 251,618, filed January 15, 1963 and entitled LowLoss Thermoelectric Heat Exchanger.

This invention relates to a thermoelectric heat exchanger and inparticular to a cooling apparatus having an intermittently operatedthermocouple.

Thermoelectric cooling systems general-1y comprise an insulated coldchamber, :a thermocouple having its cold junction thermally coupled tothe cold chamber, and a large efficient heat exchanger thermally coupledto the hot junction of the thermocouple. Such cooling systems are notefficiently energized intermittently by switching power on and off as inordinary compressor type cooling systems because when the thermocoupleis turned off after adesired cold temperature has been reached (operatedintermittently), heat flows from the ambient to the cold chamber due tothermal conduction through the thermocouple.

The objects of this invention are to provide a temperature controllingthermoelectric system that minimizes the rate of heat flow between theambient environment and the cooling chamber due to thermal conduction;that is especially adapted for use with the aforementionedthermoelectric cooling system to limit the rate of heat flow by thermalconduction from the ambient environment to the hot junction of thethermocouple when the thermocouple is not energized to -a value verysubstantially less than the total rate of heat flow from the hotjunction to the ambient environment when the thermocouple is energized;that provides efiicient operation ofan intermittently energizedthermocouple; that requires little heat pumping to overcome only smalllosses while maintaining prescribed temperatures for long periods oftime; and that will withstand power failures.

This invention is characterized by a connection between the temperaturecontrolled chamber and the ambient environment in the form of a heattransfer path, a portion of which is insulated from the ambient andadapted to transfer heat substantially exclusive of thermal conduction.

In the drawings: 1

FIG. 1 is a schematic sectional view showing a thermoelectric coolingsystem having a fan for establishing convection heat transfer from athermocouple to the ambient when the thermocouple is energized and agravity entrapment device that isolates the thermocouple from theambient when the thermocouple is not energized.

FIG. 2 is a fragmentary view illustrating a modification of the coolingsystem shown in FIG. 1 wherein louvered valves isolate the thermocoupleand heat sink from the ambient when the thermocouple is not energized.FIG. 3 is an end view of the louvered valve taken along lines 3-'3 ofFIG. 2.

FIG. 4 is a schematic sectional view showing a thermoelectric coolingsystem having a liquid-to-gas device which transfers heat from thethermocouple to the ambient when the thermocouple is energized andisolates the thermocouple from the ambient when the thermocouple Thethermoelectric cooling system shown in FIG. 1

Patented Nov. 9, 1965 ice generally comprises a cold chamber 10, athermoelectric heat pump 12, a finned heat sink 13, a bent convectionduct 14, an electrically operated fan 16 and a direct current powersupply, illustrated as battery 18. Cold chamber 10 is bounded by walls17, preferably made of high thermal conductivity material. Walls 17 andduct 14 have thermal insulation 19 encased in a protective housing 20. Asealable hinged door 21 is provided for access to cold chamber 10. Heatpump 12 comprises a plurality of thermocouples 22 connected in seriesacross battery 18 by electrical conducting strips 24, 26. Thermocouples22 are properly poled so that for a given polarity of battery 18, thejunctions 28 adjacent conducting strips 24 are cold and the junctions 30adjacent conducting strips 26 are hot. Heat sink 13 has a base plate 29,shown in broken lines, and a plurality of upstanding fins 31, the baseplate 29 being thermally connected to but electrically insulated fromconducting strips 26 by conventional means such as thin mica sheets.Conducting strips 24 are similarly insulated electrically from wall 17.Duct 14 has upwardly extending arms 32, 34 interconnected by a bight 36,bight 36 being below the upper ends of arms 32, 34 as viewed in FIG. 1.The upper ends of arms 32, 34 open to the ambient. Pins 31 of the heatsink 13 are positioned within bight 36. Duct 14 has a pair of annularsegments 40 made of insulating material so that heat is not conductedbetween bight 36 and the upper ends of arms 32, 34 by the walls of duct14. Fan 16 is positioned within arm 34 adjacent the open end thereof.Fan 16 and thermocouples 22 are connected in parallel across battery 18through an onoff switch 41 and a temperature controlled switch 42.Switch42 may be a conventional thermostatic switch providing atemperature lag between opening and closing in response to thetemperature changes in cold chamber 10 sensed by element 43.Alternatively, switch 42 may be a 'bimetal thermostatic switchpositioned within chamber 10. Any suitable circuit connection can beused to energize fan 16 and thermoelectric heat pump 12 so long as fan16 is on when thermoelectric heat pump 12 is on and the fan is off whenthe pump is off.

When on-off switch 41 is closed, if the temperature in chamber 10 isabove a prescribed cold temperature switch 42 will be closedautomatically to energize fan 16 and heat pump 12. Heat is pumped fromthe cold chamber 10 into the bight 36 of duct 14 through electricalconducting strips 24, thermocouples 22, conducting strips 26 and heatsink 13. Heat is transferred from heat sink 13 to the ambient byconvection currents established within duct 14 by fan 16. After thetemperature of cold chamber 10 is reduced to a desired level, switch 42opens automatically breaking the circuits for fan 16 and heat pump 12.With heat pump 12 off heat will flow from the mass of air in bight 36through finned heat sink 13 and thermocouples 22 back into cold chamber10 until the temperature of the air mass in bight 36 and the coldchamber 10 equalizes. The mass of air in bight 36, cooled below theambient temperature by reverse heat flow, will be entrapped by gravityadjacent heat sink 13 to check reverse heat flow from the ambientthrough thermoelectric heat pump 12 to the cold chamber 10. When thetemperature within cold chamber 10 rises to a predetermined level,switch 42 will close and the cooling process will repeat.

By properly choosing the relative mass of the components, temperatureequalization for the system can be achieved with only a one or twodegree temperature change in cold chamber 10. The thermal mass of coldchamber walls 17 should be relatively large while the thermal mass ofthermoelectric heat pump 12, heat sink 13,.and those portions of duct 14below thermal insulating segment 40 and adjacent heat sink 13 should beheld to a minimum. For example, the thermal mass of heat sijnk 13 may beminimized by using numerous thin fins 3 The air entrapment device ofFIG. 1 may be modified as shown in fragment in FIGS. 2 and 3. In themodification shown in FIG. 2 when fan 16 and heat pump 12 are off air istrapped adjacent heat sink 13 by a pair of valves 44 positioned in astraight rectangular cross section duct 45 outwardly of insulatingsegments 46. Valves 44 each have a plurality of pivoted louvered slats47 that are opened automatically toward the left as viewed in FIG. 2 byconvection within duct 45 when fan 16 is on and are closed automaticallyby gravity or suitable spring biasing (not shown) when fan 16 is off.Valves 44 may also be used with the bent convection duct 14 in FIG. 1.

In FIG. 4 the secondary heat exchanger for minimizing reverse heat flowthrough the thermoelectric heat pump is a sealed liquid-to-gas system.Elements corresponding to those in FIG. 1 are indicated by correspondingreference numerals. The sealed liquid-to-gas system comprises anevaporator 48, an outlet tube 50, a condenser 52 and return tube 54,charged With a suitable liquid refrigerant 56 to a level in evaporator48 indicated generally at 58. In the closed system liquid refrigerant 56should have a boiling point slightly above the ambient temperatureenvironment surrounding condenser 52. Evaporator 48 is encased withinsulating material 60. Cold chamber has insulation 19. Outlet tube 50and return tube 54 have thermal insulating segments indicated generallyat 62 and 64, respectively so that the entire pumping system, includingcold chamber 10, evaporator 48 and a portion of outlet and return tubes50, 54 adjacent evaporator 48, is thermally insulated from the ambient.Condenser 52 has conventional coils 66 and fins 68 and is positionedabove evaporator 48 so that when refrigerant 56 condenses it returns toevaporator 48 through return tube 54 by gravity. Preferablythermocouples 22 are potted with a suitable corrosive resistant, andelectrical and thermal insulating material 70. Cold chamber wall 17 isseparated from refrigerant 56 by thermal insulation 72. Heat sink fins74 are attached directly to conducting strips 26. However, sinceconventional liquid refrigerants, such as freon, are good electricalinsulators and have low thermal conductivity, potting material 70 andinsulation 72 could be omitted.

With the thermoelectric cooling apparatus described in conjunction withFIG. 4, when on-off switch 41 is closed, if the temperature withinchamber 10 is above a prescribed cold temperature, switch 42 will beclosed automatically to energize the thermoelectric heat pump 12. Heatpump 12 pumps heat from cold chamber 10 to liquid refrigerant 56. Thetemperature of refrigerant 56 increases until the .boiling point ofrefrigerant 56 is reached, at which point heat will be transferred fromevaporator 48 to the condenser 52 by the refrigerant in its vapor state.The refrigerant condenses in condenser 52, transferring heat to theambient, the condensed liquid refrigerant being returned to evaporator48 through return tube 54 by gravity. Heat is pumped from cold chamber10 to the ambient through the liquid-to-gas system in this manner untilthe desired temperature within cold chamber 10 is reached at which timeswitch 42 opens automatically to turn heat pump 12 off. After heat pump12 is deenergized, thermal conduction transfers heat from refrigerant 56back to cold chamber 10 until the temperature equalizes in cold chamber10, walls 17, thermoelectric heat pump 12, heat sink fins 59,refrigerant 56, evaporator 48 and those portions of outlet tube 50 andreturn tube 54 that are between the evaporator 48 and insulating segments 62, 64.

In the construction shown in FIG. 4, cold chamber 10 should have arelatively large thermal mass whereas the thermal mass of refrigerant'56, evaporator 48, heat sink fins 59 and thermoelectric heat pump 12should be maintained at a minimum so that the temperature within thesystem is equalized without appreciably reducing the temperature withinthe cold chamber 10 when the heat pump is turned off. The closed systemmay be charged with any suitable refrigerant such as freon so long asthe liquid refrigerant 56 in evaporator 48 boils and the vapor Withincondenser 52 condenses at a temperature slightly above the ambient inaccordance with techniques well known in the refrigeration arts. In aclosed evaporator-condenser system the freon vapor pressure willestablish a boiling point near ambient temperature when properlycharged.

We claim:

1. A cooling apparatus comprising a cooling chamber and means fortransferring heat from said cooling chamber to an ambient environment,said heat transferring means comprising intermittently energizedthermoelectric means and secondary heat transfer means, said secondaryheat transfer means reducing reverse heat flow in a path through saidthermoelectric means from said ambient environment to said coolingchamber when said thermoelectric means is not energized as compared toforward heat flow in the same path from said cooling chamber to theambient environment when said thermoelectric means is energized, saidsecondary heat transfer means comprising a duct for carrying a fluid, afirst portion of said duct communicating with said ambient environmentand a second portion of said duct communicating with said thermoelectricmeans, and means intermittently energized concurrently with energizationof said thermoelectric means to selectively force the fluid to flow insaid duct when said thermoelectric means is energized.

2. A cooling apparatus comprising a cooling chamber and means fortransferring heat from said cooling chamber to an ambient environment,said heat transferring means comprising intermittently energizedthermoelectric means and secondary heat transfer means, said secondaryheat transfer means limiting reverse heat flow through saidthermoelectric means from said ambient environment to said coolingchamber when said thermoelectric means is not energized, said secondaryheat transfer means comprising a gaseous medium, a duct for said medium,said duct having end portions communicating with said ambientenvironment and an intermediate portion communicating with saidthermoelectric means, said intermediate portion being below said endportions, and a fan to force said gaseous medium through said duct whensaid thermoelectric means is energized.

3. A cooling apparatus comprising a cooling chamber and means fortransferring heat from said cooling chamber to an ambient environment,said heat transferring means comprising intermittently energizedthermoelectric means and secondary heat transfer means, said secondaryheat transfer means limiting reverse heat flow through saidthermoelectric means from said ambient environment to said coolingchamber when said thermoelectric means is not energized, said secondaryheat transfer means comprising a fluid and a duct for carrying saidfluid, a first portion of said duct communicating with said ambientenvironment and a second portion of said duct communicating with saidthermoelectric means, a pump for forcing said fluid from said secondportion to said first portion when said thermoelectric means isenergized, and a valve positioned within said duct for selectivelyestablishing fluid communication between said first and second portions.

4. A cooling apparatus comprising a cooling chamber and means fortransferring heat from said cooling chamber to an ambient environment,said heat transferring means comprising intermittently energizedthermoelectric means and secondary heat transfer means, said secondaryheat transfer means reducing reverse heat flow in a path through saidthermoelectric means from said ambient environment to said coolingchamber when said thermoelectric means is not energized as compared toforward heat flow in the same path from said cooling chamber to theambient environment when said thermoelectric means is energized, saidsecondary heat transfer means comprising a duct for carrying a fluid, afirst portion of said duct communicating with said ambient environmentand a second portion of said duct communicating with said thermoelectricmeans, said first portion of said duct being thermally insulated fromsaid ambient environment and said second portion of said duct beingbelow said first portion.

5. A cooling apparatus comprising a cooling chamber and means fortransferring heat from said cooling chamber to an ambient environment,said heat transferring means comprising intermittently energizedthermoelectric means and secondary heat transfer means, said secondaryheat transfer means reducing reverse heat flow in a path through saidthermoelectric means from said ambient environment to said coolingchamber when said thermoelectric means is not energized as compared toforward heat flow in the same path from said cooling chamber to theambient environment when said thermoelectric means is energized, saidsecondary heat transfer means comprising a duct for carrying a fluid, afirst portion of said duct communicating with the ambient environmentand a second portion of said duct communicating with said thermoelectricmeans, said second portion of said duct being below said first portionand thermally insulated from said first portion.

6. A cooling apparatus comprising a cooling chamber and means fortransferring heat from said cooling chamber to an ambient environment,said heat transferring means comprising intermittently energizedthermoelectric means and secondary heat transfer means, said secondaryheat transfer means reducing reverse heat flow in a path through saidthermoelectric means from said ambient environment to said coolingchamber when said thermoelectric means is not energized as compared toforward heat flow in the same path from said cooling chamber to theambient environment when said thermoelectric means is energized, saidsecondary heat transfer means comprising a duct for carrying a fluid, afirst portion of said duct communicating with the ambient environmentand a second portion of said duct communicating with said thermoelectricmeans, said second portion of said duct being below said first portion,and a pump intermittently energized concurrently with energization ofsaid thermoelectric means for forcing the fluid from said second portionof said duct to said first portion of said duct when said thermoelectricmeans is energized.

References Cited by the Examiner UNITED STATES PATENTS 2,932,953 4/60Becket 623 3,100,969 8/63 Elfving 62-3 3,138,934 6/64 Roane 62-33,154,926 11/64 Hirschhorn 62-3 WILLIAM J. WYE, Primary Examiner.

1. A COOLING APPARATUS COMPRISING A COOLING CHAMBER AND MEANS FORTRANSFERRING HEAT FROM SAID COOLING CHAMBER TO AN AMBIENT ENVIRONMENT,SAID HEAT TRANSFERRING MEANS COMPRISING INTERMITTENTLY ENERGIZEDTHERMOELECTRIC MEANS AND SECONDARY HEAT TRANSFER MEANS, SAID SECONDARYHEAT TRANSFER MEANS REDUCING REVERSE HEAT FLOW IN A PATH THROUGH SAIDTHERMOELECTRIC MEANS FROM SAID AMBIENT ENVIRONMENT TO SAID COOLINGCHAMBER WHEN SAID THERMOELECTRIC MEANS IS NOT ENERGIZED AS COMPARED TOFORWARD HEAT FLOW IN THE SAME PATH FROM SAID COOLING CHAMBER TO THEAMBIENT ENVIRONMENT WHEN SAID THERMOELECTRIC MEANS IS ENERGIZED, SAIDSECONDARY HEAT TRANSFER MEANS COMPRISING A DUCT FOR CARRYING A FLUID, AFIRST PORTION OF SAID DUCT COMMUNICATING WITH SAID AMBIENT ENVIRONMENTAND SAID DUCT COMMUNICATING SAID DUCT COMMUNICATING WITH SAIDTHERMOELECTRIC MEANS, AND MEANS INTERMITTENTLY ENERGIZED CONCURRENTLYWITH ENERGIZATION OF SAID THERMOELECTRIC MEANS TO SELECTIVELY FORCE THEFLUID TO FLOW IN SAID DUCT WHEN SAID THERMOELECTRIC MEANS IS ENERGIZED.